Adjustable T-squares typically comprise a frame having a generally flat working surface with a measuring blade slidably mounted on the frame at the end of the working surface and normal to the working surface. Such devices often comprise a level vial mounted on the frame in parallel relation to the working surface.
Conventional adjustable T-square devices are suitable for checking the accuracy of external right angle corners formed by two abutting objects such as pieces of wood, but generally cannot be used to check the accuracy of internal corners formed by such objects, particularly if the internal corner is rounded.
If it has a level vial, the T-square can be used to check deviations from true horizontal of a surface but can only check deviations from vertical of a generally vertical surface if that surface forms an external right angle with a true horizontal surface. Further, there are many corners formed by angles other than right angles, particularly at 45.degree. angles, whose accuracy cannot be checked by conventional adjustable T-squares.
Accordingly, there is a need for an adjustable T-square which can be used on internal corners as well as external corners, which can be used with angles other than right angles and which can accurately check the accuracy of vertical and horizontal surfaces.